Liquid level measuring devices have been known for many years. Their purpose is to locate the level of a flowable material, or to indicate the amount of flowable material remaining in a container.
On many occasions, monitoring the amount of flowable material in a container is required. However, direct observation of the flowable material level is not always possible or practical. Measurement of the material in such containers as pressurized cylinders, sealed containers, cryogenic flasks, and opaque vessels is often difficult. Such measurements are even more troublesome when the material within the container is corrosive or potentially toxic or flammable.
Sight glasses and weight scales are some examples of liquid level measuring devices which are commonly employed. Both of these devices suffer from a number of disadvantages. Sight glasses are expensive, and they can crack and break easily. On such occasions where the container is placed outdoors, ultraviolet light can cause the glass to haze. Weight scales are also expensive, and in many instances, measurements provided by weight scales are inexact.
A simple, economical external liquid level gauge which permits a direct reading of the level of a flowable material is taught in RAIT Canadian Patent No. 1,177,281 issued on Nov. 6, 1984. The liquid level measuring device taught therein employs one thermochromatic material which is coated onto a base layer. The base layer is magnetically mounted to the outside surface of the outside wall of the container, and thus the external liquid level gauge can be repeatedly removed and replaced or relocated when necessary.
The theory is that the rate of heat transfer is different between a mass of flowable material and the void volume above it such that for any container with a modest heat conducting capability, the container wall experiences a temperature gradient which is most pronounced at the interface of the contents with the void volume above the contents, and of course below that interface. That is to say, the rate of heat transfer through the wall of a container will be greater where there is a mass of flowable material located in the container than where there is a void volume above the flowable material. In other words, the rate of heat transfer through the container wall changes most abruptly at the level of the interface, and below. Thus, with the use of a thermochromatic material, a vivid color change occurring at the interface, and below, will permit an observer to obtain a direct reading of the level of the flowable material within a container by discerning where the interface is located.
RAIT U.S. patent application Ser. No. 10/077,971 filed Feb. 20, 2002, for “External Liquid Level Gauge,” teaches an external liquid level gauge which is adapted to be affixed vertically to the outside wall of a container. The external liquid level gauge as taught therein is in the form of an elongated strip and it comprises a layer of base material and a layer of thermochromatic materials. Furthermore, the thermochromatic layer comprises a light absorbing background and at least two regions of thermochromatic materials which are arranged upon the light absorbing background. The regions of at least two thermochromatic materials are disposed in arrays thereof and are arranged entirely along the length of the external liquid level gauge. Moreover, each of the thermochromatic materials responds chromatically within a different operating temperature range.
Several other prior art thermochromatic external liquid level gauges are now described. They include GILMOUR U.S. Pat. No. 3,696,675 issued Oct. 10, 1972, which teaches an external liquid level gauge adapted to be permanently affixed to the outside wall of a container for determining the liquid-gas interface within the container. The external liquid level gauge described therein consists of a uniform thermochromatic liquid crystalline material which coats the entire base layer of the gauge such that it is at right angles to the liquid-gas interface. The uniform thermochromatic material covers the entire temperature range to which the container is subjected within an overall range of −20° C. to 250° C. Depending upon the thermochromatic material selected, color changes over a gradient from violet to red can occur in a range from as small as 2° C. to one as broad as 150° C. Since the temperature differential across the liquid-gas interface is generally small, on the order of less than 2° C., the change in color is slight across the interface. This is particularly the case when the container is placed outdoors and a large temperature range needs to be covered. As a result, it is difficult to visually locate the liquid-gas interface.
In U.S. Pat. No. 5,323,652 issued Jun. 28, 1994 to PARKER, the inventor teaches a thermochromatic level indicator for determining the level of a material inside a container. The thermochromatic level indicator includes at least two thermochromic materials of different opacities and transition temperature. Prior to the attachment of the thermochromatic level indicator to the outside surface of the outside wall of the container, the thermochromic materials are applied to a transparent film by silk screening, other printing and coating methods, or methods which employ the use of microencapsulated thermochromic materials. The thermochromatic level indicator may be permanently adhered to the container wall or it may be adhered to a magnetic strip which can be temporarily affixed to the container wall.
In another U.S. Pat. No. 5,707,590, issued Jan. 13, 1998, the inventor THOMAS et al has provided a detergent container with a thermochromatic level indicator. In one embodiment of the invention, the thermochromatic substance is added to the container's plastic material during the molding process. In another embodiment of the invention, the level indicator or strip comprises a base material, such as Mylar, which is coated or embedded with a thermochromatic substance by such methods as painting, stripping, or screen printing.
However, many occasions arise where it is not convenient or impractical to go to the site where a storage tank or tanks are located, and in which fluid—usually a liquid such as liquid gases including propane and the like—may be stored. For example, large storage tanks may be remotely located to supply heating fuel to likewise remotely located automatic weather stations, livestock shelters, storage depots, and the like. Those locations are typically accessible by road, rail, floating tank vessels, or helicopter, to replenish the supply of fuel, but the timing of such refuelling visits may not necessarily be periodic. That is, the vagaries and requirements of weather and climate may result in more or less consumption of fuel, and faster or slower exhaustion of the fuel supply. Obviously, it is not economical to visit a remote site for refuelling purposes if the storage tank for the fuel has only been exhausted by, say, 25% of its capacity. Equally obviously, a refuelling visit must be made before total exhaustion of the fuel supply at the remote location.
Accordingly, it is desirable to provide a level detector for storage tanks for fluids that can be remotely operated, or at least that can function and provide data indicative of the level of fluid storage in a storage tank without on-site human intervention. Accordingly, any level indicator which relies on a visual indication is not at all useful. Moreover, it is the intent and purpose of the present invention to provide level detectors for storage tanks and the like which are external, and therefore do not rely on float and valve assemblies and the like, and which can therefore also be applied to a wide variety of storage tank structures.
The present invention is intended to function so as to provide an approximation of the fluid level within a storage tank. As will be seen, particularly when remote storage tanks are considered, it is unimportant to be exact, provided that an approximation to within at least a few percent of the actual fluid level within the storage tank can be arrived at.
The inventors herein have unexpectedly discovered that it is quite possible to take advantage of the theory of the rate of heat transfer being different between a fluid such as a liquid, and the void volume above it, for any container which has at least a modest heat conducting capability, where such theory may be exploited remotely as a consequence of the use of elements or material which have high temperature coefficients. Moreover, the present inventors have unexpectedly determined that by appropriate spacing of heating elements vertically along the wall of a storage tank, and by applying appropriate sampling techniques to determine the difference between the rate of heat loss by conduction from various previously heated elements arranged vertically along a storage tank wall, a quite reasonable approximation of the fluid level within the storage tank can be determined.
All of this is possible because elements and materials exist that do, indeed, have appropriate high temperature coefficients; and because remote control of sampling and data communication is easily achievable.
For example, a remote location might, indeed, be connected at least by wire or wireless means into a network, a specific URL, wireless radio identity, mobile or cellular telephone number, or other electronic identity, so that it may be polled from time to time. Such polling would instruct that a level detection procedure should proceed alternatively, or as well, any remote location can be set up and programmed so that it will, on its own, periodically “wake up” and perform a level detection procedure as described hereafter.
By the provision of battery operated electronic and electrical apparatus, the present inventors have been able to provide a level detector for storage tanks for fluids that is remotely located, and which may function periodically or on demand, requiring visits to the remote location only when it is necessary to refill the storage tank. Typically, the battery life of batteries that are on site at the remote location is designed and expected to be much greater than the anticipated interval between refilling visits, but nonetheless the batteries can be exchanged for new ones each or every few refilling visits since the cost of replenishing a battery is minuscule when compared to the cost of refilling the storage tank.
Accordingly, by being able to monitor the differential cooling of the wall of a storage tank as a consequence of the difference in thermoconductivity and heat capacity of the fluid, typically a liquid, within the tank as opposed to the gas in the void above the fluid level within the tank, level detection is relatively easily achieved in keeping with the concepts and principles of the present invention.
Essentially, the present invention provides for a network of resistive elements, each having a high temperature coefficient, to be placed in a series or parallel connection across a high power source and to be heated by passing a relatively high current through the resistive elements. Then, a relatively low current may be passed through the same network of resistive elements, by employing a low power source, so that the power delivered to the network of resistive elements is lower than that which would cause additional heating effect of the resistive elements, but is sufficient to permit voltage detection at low resistivity, conductive elements between the resistance elements.
In other words, during a detection procedure after a heating step, the network of resistive elements can be considered to be effectively a voltage divider because the resistive elements which are below the level of the fluid within the tank will lose heat by conduction more quickly to the fluid than will the resistive elements that are above the level of fluid within the tank. Thus, their resistance will either decrease or increase at a commensurately greater rate, depending on whether or not the resistive elements have a positive temperature coefficient or a negative temperature coefficient.
It is recognized that the wall of the tank is, itself, heat conductive. However, if there are localized areas of heat that are arranged vertically along the wall of the tank, then each of those localized areas of heat will lose heat due to a combined effect of heat conduction as a consequence of the heat conduction characteristic and capacity of the wall of the tank per se, plus the heat conduction characteristics of the fluid or gas which is located at the same horizontal level as each of the vertically arranged resistive elements or heat spots.
Obviously, heat will be lost through conduction much faster below the level of the fluid within the tank, as a consequence of the higher heat capacity of the fluid, than it will above the fluid level as a consequence of the much lower heat capacity of the gas or void within the storage tank—the heat capacity of the wall of the storage tank being the same in any location and therefore permissible to be ignored for the purposes of the present invention.
Provided that the distance between the heating elements is greater than the thickness of the material of the storage tank, then the differential cooling rates of heat elements above and below the fluid level within the storage tank will come into play, and the differential rates of loss or increase of resistivity due to cooling will be detectable, so that it will be possible to make a reasonable approximation of the fluid level within the storage tank.
By employing a plurality of resistive elements each of which has a high positive or negative temperature coefficient, and each of which is such that as its entrained heat decreases due to heat loss because of conduction, its electrical resistance reduces or increases, and provided that it is possible to sample the voltage at the junction between each adjacent pair of resistive elements, then as a consequence of the network connection of resistive elements functioning effectively as a voltage divider, an approximation of the fluid level within the storage tank will be determined.
Typically, but not necessarily, the resistive elements are such as to have a positive temperature coefficient, whereby their resistance value increases as they are heated. Also, typically, but not necessarily, the network of resistive elements is such that they are connected in series.
It follows also that the size of the resistive elements must be greater than the thickness of the wall of the storage tank, as well as the spacing between the resistive elements. This is so as to provide sufficient heat to the immediate area or region where the resistive element is located, so as to raise the local temperature at that location significantly above ambient temperature.
Obviously, the invention as it has so far been described will function quite well with a series of thermisters. They may be first heated and thereby function as power resistors, and then measured until such time as the cooling rates of the thermisters separate them into two groups, a fast cooling group which is below the fluid level within the storage tank, and a slow cooling group which is above the fluid level within the storage tank, as mentioned above.
However, thermisters are expensive elements, and may require significant power in order to be heated in the first instance. That may require, therefore, a reasonably significant capital cost in respect of the provision of a plurality of thermisters and the provision of a power source sufficient to supply heating power to the thermisters to heat them up.
A further provision of the present invention, however, comes as a consequence of the unexpected discovery that a number of resistive inks are available which will meet the requirement of a resistive element having a high temperature coefficient, so that resistive elements can be effectively printed using resistive inks. Thus, if the resistive elements are connected with low resistivity conductive elements between them, then a high electric current which is passed through the resistive elements will cause the elements to become heated, and their resistance to increase or decrease; but when a low electric current is passed through them after they are heated then their heat will be given up through heat conduction as discussed above, and their resistance will decrease or increase, depending on whether they have a positive temperature coefficient or a negative temperature coefficient.
From all of the above, it follows that relatively little resolution is required to determine the fluid level within a storage tank, and an approximation is quite sufficient because a decision whether or not to refill the storage tank after any procedure to determine the fluid level within that storage tank has been carried out is made when necessary, over a relatively broad percentage of depletion of the stored fluid within the tank.
The provision of apparatus in keeping with the present invention is made easier as a consequence of the relatively low cost and ease of programming of microprocessors. In other words, a microprocessor can be provided and programmed so as to periodically cause a level detection procedure to occur, or to respond to a remote requirement for the level detection procedure to be undertaken. During the lengthy periods of time between level detection procedures, there is very little power demand by the microprocessor on the power supply, so that sleep mode consumption of battery power is effectively irrelevant in terms of calculation of expected battery life.
If an area of the resistive ink is applied to a suitable substrate, such as polyaramide, where the substrate functions effectively as a flexible printed circuit having copper between the resistive lands created by the areas of resistive ink, then a very simple level detector in keeping with the present invention may be provided that can simply be adhered to the outer wall of a storage tank. It will be appreciated that the thermal resistance of a thin layer of adhesive will be small compared to that of the material of storage tanks—usually steel. Moreover, the electronics by way of a microprocessor and control circuitry and the like, can be bonded directly to the substrate, so that the only other element which is required is a power supply which will typically comprise a battery pack. Thus, the provision and maintenance of level detectors in keeping with the present invention may be effected quite easily and economically.